To do this mission you have to find 4 clues stratched on walls around Los Santos. You have to play with Michael.
The final piece of the puzzle can be found on the desk of Solomon Richards. To access it, you need to have completed the Meltdown mission then head to the marker outside his office at Richards Majestic after 21:00. You can then read the letter on his desk, revealing a dark secret hidden in a mine up in the hills.
Out in Great Chaparral you’ll find an abandoned mine, where you can use explosives to blow the doors off and gain access. Follow the rail tracks into the mine and you’ll eventually discover a body in a pinstripe suit, which should unlock the option for new Vintage filters.
With the Murder Mystery solved, you can now modify your display with two new Vintage Noir filters. These can be accessed by visiting the body in the mine, or by calling the new Isaac contact on your phone to cycle through them.